The MRI (magnetic resonance imaging) apparatus utilizes the NMR phenomenon to measure a density distribution, a relaxation time distribution, and the like of nuclear spins in a desired portion under testing of the object, and displays an image of a cross-section of an arbitrary portion of the object from the measured data.
The technology for producing a blood flow image utilizing such a magnetic resonance imaging apparatus is called MRA (magnetic resonance angiography). This MRA can be roughly classified into an approach which uses a contrast medium and an approach which does not use the contrast medium.
Here, a portion supplied with the contrast medium can receive a larger signal, as compared with a portion not supplied with the same, so that a vertical relaxation time T1 can be reduced.
Known as approaches for the MRA which do not use the contrast medium include a time-of-flight (TOF) method which utilizes a flow-in effect into a slice plane to extract a blood flow image; a phase-sensitive (PS) method which utilizes the presence or absence of phase diffusion by a blood flow to perform differential processing for extracting a blood flow image; a phase-contrast (PC) method which performs differential processing for inverting the polarity of phase diffusion by a blood flow to extract a blood flow image.
These approaches are described in detail in “Magnetic Resonance Imaging; Stark D D et al. edited, The C. V. Mosby Company, pp108–137, 1998”; Keiji Fukui et al., “Research on Cerebral MRA Angio-imaging—First Report—,” CT Research, 10(2) 1998, pp133–142; “Magnetic resonance angiography, Dumoulin C L, et al., Radiology 161: 717–720, 1986; “Simultaneous acquisition of Phase-contrast angiograms and stationary-tissue images with Hadamard encoding of flow-induced phase shifts,” Dumoulin C L, et al, J. Magnetic Resonance Imaging 1: 399–404, 1991; “Encoding strategies for three-direction Phase-contrast MR imaging,” Dumoulin C L, et al., J. Magnetic Resonance Imaging 1: 405–413, 1991; and the like.
On the other hand, an MRA approach using a contrast medium typically combines a T1-reducing contrast medium such as Gd-DTPA and a gradient echo based pulse sequence having a short repetition time TR. This approach repeats excitation in the same region by a high frequency magnetic field at short time intervals of several milliseconds to several tens of milliseconds to saturate spins in portions other than blood flows, and captures high signals from blood flows, in which T1 is reduced due to the contrast medium contained therein, and saturation of spins is hard to occur, to extract a blood flow image.
Further, a DSA approach (Digital Subtraction Angiography) which takes a difference between an image before shadowing and an image after shadowing provides an image which shows blood flows at a higher contrast. Here, in the MRA approach which uses such a contract medium, the contrast medium is typically injected from an elbow vein. The injected contrast medium circulates the heart, arterial system, capillary blood vessels, and vein system in sequence.
Thus, a technology called “dynamic MRA” repeats measurements in each step in the circulation of the contrast medium to produce time-series images of blood flows in respective regions.
The MRA approach and dynamic MRA using such a contrast medium are described in detail in “3D Contrast MR Angiography,” 2nd edition, Prince M R, Grist T M and Debatin J F, Springer, pp3–39, 1998, while the dynamic MRA is described in detail in pp16–19 of this literature.
An imaging time for one session in the dynamic MRA as described above is the product of a repetition time TR and the number of phase encoding steps for two-dimensional measurements, and the product of the repetition time TR, the number of phase encoding steps, and the number of slice encoding steps for three-dimensional measurements.
It is therefore desirable to increase the number of phase encoding steps and the number of slice encoding steps for improving the spatial resolution. On the other hand, an increased number of phase encoding steps or slice encoding steps results in a lower temporal resolution. In other words, the spatial resolution and temporal resolution are basically in a trade-off relationship.
To solve such a problem, U.P. Pat. Nos. 5,713,358 and 5,830,143 propose the following measuring methods.
Specifically, these techniques do not measure an overall k-space (space under measurement) in one measurement, but divide the k-space into a plurality of unit regions in a ky-direction, where kx, ky, kz represent three-dimensional directions in the k-space, and control divided regions of the k-space to be measured in each session such that a central region of the k-space (low frequency region) is measured more frequently.
Then, the result of a measurement made in another session is diverted to a region of the k-space which was not measured in a certain session. Alternatively, it is produced through interpolation from the results of a plurality of measurements made in other sessions.
Since the foregoing techniques eliminate the need for executing all phase encoding steps in each session of measurement, they can reduce a measuring time in each session, improve the temporal resolution, and exactly capture a change over time in the central region (low frequency region) of the k-space which relatively determines the contrast important for diagnosis.